Circuit for energizing electromagnetic operated hammers in a high speed impact printer



United States Patent 3,519,893 CIRCUIT FOR ENERGIZING ELECTROMAGNETIC OPERATED HAMMERS IN A HIGH SPEED IM- PACT PRINTER Harold S. Schwartz, White Plains, N.Y., assignor to Potter Instrument Company, Inc., Plainview, N.Y., a corporation of New York Filed Sept. 29, 1967, Ser. No. 671,752 Int. Cl. H01h 47/32 US. Cl. 317-1485 4 Claims ABSTRACT OF THE DISCLOSURE An impact printer has solenoid operated print hammers which are fired by a pulse on the control electrode of a silicon controlled rectifier in series with the solenoid operating coils. A transistor switch in series with the cathodes of two or more SCRs is provided to turn them off, and a resistor coupling the cathodes to the power supply is provided to reverse bias the SCRs and to insure turn off. A capacitor coupled to the SCR anode holds it at a low potential following turn off, preventing noise and allowing a sufficient interval for reverse biasing the SCR.

BACKGROUND OF THE INVENTION This invention relates to high speed impact printers which employ solenoid operated print hammers and, more particularly, to an improved silicon controlled rectifier (SCR) hammer operating circuit for such printers.

One object of this invention is the provision of a simple, economical SCR print hammer operating circuit which has a low electromagnetic interference (noise) output.

Another object of the invention is the provision of an improved SCR print hammer operating circuit for impact printers which reduces the number of components as compared with prior art SCR hammer operating circuits.

One more object of the invention is the provision of an improved SCR hammer operating circuit which is relatively immune from noise.

A further object of the invention is the provision of an improved SCR hammer operating circuit which can operate over a wide temperature and humidity range with commercially available electronic components.

SUMMARY OF THE INVENTION Briefly, this invention contemplates a solenoid operated print hammer firing circuit in which an SCR switch is in series with the operating coil of each print hammer. A single transistor switch, coupled in series with the cathodes of several of the SCRs, extinguishes conduction through them by cutting off the flow of current through them, and a resistor coupling the cathode to the power supply back biases the SCRs insuring that conduction is extinguished. A capacitor is coupled to the anode of each SCR to provide a conduction path for the coil following turn off of the SCR and to thereby limit the rate of change of current and voltage following this turn off, reducing the noise generated. In addition, this capacitor holds the anode of the SCR at a low potential for an interval sufficient to allow dissipation of the charge accumulated by the SCR during conduction.

BRIEF DESCRIPTION OF THE DRAWINGS Having briefly described this invention, it will be described in greater detail along with other objects and advantages in the following detailed description of a preferred embodiment which may be best understood by reference in the accompanying drawing which forms part Patented July 7, 1970 DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, the printer has a row of print hammers 12 disposed adjacent a rotating drum 10. Only the first and last hammers are shown in order to simplify and clarify the drawing. Each print hammer 12 has a solenoid operating coil 14 which, when energized, causes its hammer to fire.

Although the invention is applicable both to parallel printers in which more than one hammer at a time may fire and character-at-a-time printers in which the columns are printed one at a time from right to left, for example, the invention will be explained particularly with reference to the latter type of printer.

The positive terminal of a direct current (DC) power supply 28 is coupled to one side of each of the coils 14 via current limiting resistors 32. The other side of each coil 14 is coupled respectively to the anode of an SCR 26. In the character-at-a-time printer of this embodiment of the invention, the collector of a single NP-N transistor switch 34 is coupled to the cathodes of all the SCRs 26. When this transistor is in saturation, it couples the cathodes of the SCRs 26 to ground. When transistor 34 is cut oil, it prevents the flow of current through the SCRs and thereby extinguishes any previously fired SCR. In a parallel printer, a single transistor switch 34 may serve a group of hammers, the number of hammers in the group depending upon the current carrying capability of the transistor.

Coupled to the drum 10 is an encoder 16 the output of which is coupled to a logic and control circuit 18 whose other input is from a data buffer register 22, for example. As is well known in the art for character-at-a-time printers, each time there is a correspondence between a type face on the drum 10 and the information to be printed in a certain column, the logic and control circuit 18 produces a positive output pulse on a selected one of the lines 21 which is coupled via resistor 23 and diode 25 to the control electrode 24 of a selected SCR 26. This pulse fires the SCR.

Each of the solenoid operating coils 14 is shunted by a network comprising a capacitor 52, a resistor 54 and a diode 55. After an SCR has turned 011, this network provides a path through which current from coil 14 can continue to flow, gradually diminishing as the capacitor 52 discharges. In this way, the rate of change of current flow through coil '14 following turn off is reduced, thereby reducing the voltage spikes (noise) generated across the coil 14. It should be noted that diode 55 provides a relatively high impedance path when the SCR 26 turns on so that the coil 14 is not shunted appreciably by the capacitor 52 at turn on. It should also be noted, that alternatively, the capacitor 52 may be coupled between the anodes of SCR 26 and ground rather than the anode and the positive terminal of the power supply, as shown.

In addition to limiting the rate of change of current through coil 14 following turn off of SCR 26, the network of diode 55, capacitor 52 and resistor 54 maintains the potential at the anode of SCR 26 at a low potential for an interval suificient to permit the SCR to be back biased and to discharge the charge it accumulated during conduction. A resistor 58 couples the cathodes of the SCR 26 to the power supply 28 in order to back bias the SCR and provide a discharge path for the charge accumulated. Thus, conduction through an C 26 .Will be in ui eve c f 1e si rrnt 'of transistor 34 exceeds the Holding current of SCR 26.

transistor 38 is cut off. When transistor 38 is in saturation, it couples the control electrodes to negative bias source 42, preventing any non-conducting SCR 26 from being fired by a noise signal during the interval that its cathode is coupled to ground via transistor 34.

Negative bias sources 36 and 44 respectively bias transistors 34 and 38 to cut off; a lead 31 couples the bases of both these transistors to logic and control circuit 18 which produces a positive output pulse of fixed duration on lead 31 each time it produces a hammer firing pulse on lead 31.

In operation, when a type face on the drum is properly disposed with respect to a selected hammer 12 and the information from source 22 calls for printing that character in the selected column, a positive pulse from logic and control circuit 18 on an appropriate one of the lines 21 fires the SCR 26 in series with the selected hammer 12. Simultaneously, a positive pulse of fixed duration on line 31 drives transistors 34 and 38 into saturation.

After the hammer 12 fires, the positive pulse on line 31 terminates, cutting off transistor 34, and thereby terminating the flow of current through SCR 26. However, current from coil 14 continues to flow via diode 55 and resistor 54. This current diminishes gradually as capacitor 52 is discharged, reducing the amplitude of the voltage generated (noise) across the coil 14. In addition, this network limits the rate at which the anode voltage of the SCR rises, permitting discharge of the SCR 26 and insuring complete cut off of the SCR.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. It is further obvious that various changes may be made in details within the scope of the claims without departing from the spirit of the invention. It is, therefore, to be understood that this invention is not to be limited to the specific details shown and described.

What is claimed is:

1. In a high speed impact printer having a plurality of solenoid operated print hammers, a firing circuit for such print hammers comprising, in combination:

a plura1ity of silicon controlled rectifiers each having an anode, a cathode, and a control electrode,

a power supply,

means coupling the solenoid operating coils for the plurality of print hammers between one terminal 'of said power supply and the anode of one of said SCRs respectively,

a plurality of capacitors,

means respectively coupling each of said plurality of capacitors to each of said anodes to provide a path for current flowing through said solenoid coils following turn oif of said SCRs,

a transistor switch in series with the cathodes of said SCRs for providing a low impedance path for SCR current when said transistor switch is turned on and for providinga high'impedaiic'e path when said switch is turned off, and

a resistor coupling said cathods to said one terminal of said power supply in order to back bias said SCRs when said transistor switch is turned off.

2. In a high speed printer as in claim 1, a hammer firing circuit also as in claim 1 further including a single transistor switch in series with the cathodes of at least two SCRs for extinguishing conduction through said SCRs.

3. In a high speed printer as in claim 2, a hammer firing circuit also as in claim 2 further including means for biasing the control electrodes of the SCRs whose cathodes are coupled to said single transistor switch at a first potential when said transistor switch is conducting and at a second potential when said transistor switch is non-conducting.

4. In a high speed printer as in claim 3, a hammer firing circuit also as in claim 3 wherein said means for biasing includes a transistor switch which turns on and off in synchronism with said single transistor switch.

References Cited UNITED STATES PATENTS 3,151,311 9/1964 Spector et al. 3,243,665 3/1966 Fayer et al. 317148.5 X 3,354,358 11/1967 Bray et al. 317l48.5

I D MILLER, Primary Examiner W. I. SMITH, Assistant Examiner U.S. C1.X.R. 101-93; 3 l7l37 

